Chapter 12. Tuples and Arrays

Goal

By the end of this chapter you will be able to group values by position, in two different ways: a tuple and an array.

A tuple: a fixed group of values

A struct names its parts. Sometimes a small group of values does not need names, only an order. A pair of values, written in parentheses, is a tuple:

let event = (60, 4);

That tuple holds a pitch and a beat count, in that order. The parts of a tuple are reached by position, starting at zero: event.0 is 60 and event.1 is 4.

A tuple can also be taken apart into named bindings in one step. This is called destructuring:

let (pitch, beats) = event;

After that line, pitch is 60 and beats is 4.

An array: a fixed-length row of one type

An array is a row of values, all of the same type, with a length fixed when the program is written. It is written in square brackets:

let scale = [0, 2, 4, 5, 7, 9, 11, 12];

An entry is read by its position, again starting at zero, as scale[2]. The type of that array is written [Word; 8], meaning eight Word values.

The length is part of the type and never changes. An array does not grow or shrink. This is what makes its memory use known in advance, which Chapter 1 noted as one of the language’s promises.

A program using both

fn main() -> Word {
    let event = (60, 4);
    let (pitch, beats) = event;
    let scale = [0, 2, 4, 5, 7, 9, 11, 12];
    pitch + scale[2] + beats
}

Run it with keleusma run. The output is:

The tuple event is destructured into pitch, which is 60, and beats, which is 4. The array scale holds the major-scale step pattern from Chapter 3, and scale[2] is its third entry, 4. The sum is 60 + 4 + 4, which is 68.

Tuple or struct, array or enum

A tuple and a struct both group values that are present together. Reach for a struct when the parts deserve names, and a tuple when a short, ordered group is clearer without them.

An array holds many values of one type. An enum holds one value out of a fixed set of types. They are not alternatives to each other. They answer different questions.

What you now know

A tuple, (a, b), groups values by position, read with .0, .1, and so on, or destructured with let (a, b) = ....
An array, [a, b, c], is a fixed-length row of one type, read with array[index].
An array’s length is fixed and is part of its type.
An index that points past the end is handled with the indexing construct array[i] { ok(v) => ..., invalid_index(idx) => ... }. See Chapter 23.

The next chapter studies match, the tool for choosing on the shape of a value, in depth.

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